This proposed research has three major objectives: First,preliminary research demonstrated a real difference in the incidence of regression to RSV induced tumors between an inbred and an outbred line of chickens. Subsequently, lines of chickens have been characterized, or partially so, with respect to incidence of tumor regression. One objective is to investigate the possible influence of genes at the B and C blood group-histocompatability loci on the incidence of regression. Inbred animals will be classified according to blood group genotype and response to RSV inoculation to determine whether pattern of tumor growth is associated with blood group genotype. Second, individual tumor cases are monitored clinically by the serum blocking test. In the proposed individual chickens of known blood group genotype will be monitored by the serum blocking test to determine whether serum blocking, or unblocking, and tumor size are associated with blood group genotype. Host sera will be examined with both the blocking and the unblocking tests to determine whether or not there is an association between the blocking factor and the staining of lymphocytes with acridine orange. Third, in preliminary research it was found that acridine orange stain could be used to differentiate lymphocytes from tumor-bearing vs. non-tumor bearing chickens. A portion of the lymphocytes from the former animals produced a red fluorescence at the periphery of the lymphocyte while those from the latter demonstrated only a green fluorescence. The objective will be to clarify the origin and nature of the material coating lymphocytes and which hinders their cytotoxic activity to tumor cells.